How to reverse direction of object along path
The animated path has a parameter called "Evaluation Time". This shows in percents where is the constrained object along the path. When you click on "Animate Path" button (at the constraints menu, you already did it) the curve gets an animation modifier what you can find in the graph editor. After you delete it, you can animate the Evaluation time as any other parameter. So you can increase or decrease its value by inserting keyframes at the appropriate frame.